1. Field of the Invention
This invention relates to methods of producing high quality lubricating oil stocks, particularly to methods of producing high quality lubricating oil stocks having high VI by hydroprocessing low quality crude feedstocks.
2. State of the Art
World crude oil supply constraints are requiring refiners to use poorer quality crude oils to produce high quality lubricating oils. High quality lubricating oils must have a high viscosity index (hereinafter VI), low volatility, good low temperature fluidity, and high stability. Some of these properties can be achieved by solvent refining certain high grade crude oils, but these crude oils are becoming less available and more expensive.
The poorer quality crude oils remaining tend to have higher concentrations of aromatic compounds and asphaltenes in the heavier portion of the feedstock containing the components of the appropriate weight to produce neutral base stocks and bright stocks. In hydrocracking, the desired reactions are the saturation of polyaromatics and the opening of polynaphthenic molecules into branched paraffinic molecules. Hydrodewaxing essentially selectively hydrocracks normal paraffins, reducing the molecular weight and length of the molecules. Heavy hydrocarbon stocks, herein defined as those boiling above 650.degree. F., can be processed by hydrodewaxing to produce acceptable lubricating oil base stocks by reducing the molecular weight range of normal paraffins to below the molecular weight range of neutral stocks. Therefore, a poorer quality crude oil can be upgraded to make an acceptable lubricating oil base stock by a combination of hydrocracking and hydrodewaxing.
Poorer quality crude oils are theoretical candidates for a new source of lubricating oil base stocks. However, distillation of such crude oils normally produces poor quality lubricating oil base stocks fractions. The lubricating oil base stocks produced have an unacceptably high concentration of aromatic and naphthenic components, and, consequently, unacceptably low VIs. Hydroprocessing must be used to produce lubricating oil base stocks low in aromatic and naphthenic components. But if commercially acceptable hydroprocessing conditions are employed, then a number of difficulties will be encountered. Among the difficulties is that hydroprocessing the crude oil to remove aromatic components producing a product containing high concentrations of naphthenic components. Naphthenic components are known to degrade the VI of the resulting lubricating oil base stocks. Removal of the naphthenic components by hydroprocessing requires high temperatures and pressures. Furthermore, aromatic components tend to consume large amounts of hydrogen during hydrogenation. If these difficulties could be overcome, then a significant advantage would be gained. Then high quality lubricating oil base stocks could be produced from poorer quality crude oils using commercially acceptable hydroprocessing conditions.